1. Design Space for Combustor
P M V Subbarao
Professor
Mechanical Engineering Department
Selection of Geometry to Proved Safe, Reliable & Efficienct Combustion ….

2. Simple Bunsen Burner
Burning Velocity
Flow velocity
Air
Fuel

3. Sflamelet

4. Stability & Flammability Limits
Burning Velocity > Flow Velocity : Flash Back Limit
Burning Velocity < flow Velocity : Blow Off Limit
Burning Velocity = Flow Velocity : Stable Flame.
Rich Mixture
Fuel Flow rate
Flash Back
Stable Flame
Blow off
Lean Mixture
Air Flow rate

5. Generalized Flammability Map

6. Design Constraints: Mass Flow Rate

7. Design Constraints: Flow Velocity
Region of Stable Burning

8. Design Constraints: Mixture Temperature
Saturation Line
Rich Mixture
Spontaneous Ignition
Flammable mist
Flammable Vapour
Lean Mixture
Flash Point
SIT of Aviation fuels: 501 – 515 K
Mixture Temperature

9. Macro Design Parameter for Combustor
The ability of the combustion process to sustain itself in a continuous manner is called Combustion Stability.
Stable and efficient combustion can be upset by too lean or too rich mixture.
This situation causes blowout of the combustion process.
The effect of mass flow rate, combustion volume and pressure on the stability of the combustion process are combined into the Combustor Loading Parameter (CLP), defined as
n ~ 1.8

10. Combustion Stability Characteristics
Unstable
Stable
Unstable
CLP

11. Design Charts for Combustor Sizing

12. Design Charts for Combustion Zone Sizing

13. Geometrical Anatomy of Jet Engine Combustor

14. Special Geometries for Combustor

15. Types of Combustors

16. Combustion Design Considerations
Cross Sectional Area: The combustor cross section is determined by a reference velocity appropriate for the particular turbine.
Another basis for selecting a combustor cross section comes from thermal loading for unit cross section.
Length: Combustor length must be sufficient to provide for flame stabilization.
The typical value of the length – to – diameter ratio for liner ranges from three to six.
Ratios for casing ranges from two – to – four.
Wobbe Index: This is an indicator of the fuel characteristics and stability of the combustion process.

17. Design Constraints
Pressure Drop: The minimum pressure drop alllowed is upto 4%.
Volumetric Heat Release Rate: The heat-release rate is proportional to combustion pressure.
Actual space required for combustion varies with pressure to the 1.8 power.

18. Length Scaling
An estimate of the size of main burner is required during the engines preliminary design.
The cross sectional area can be easily determined using velocity constraints.
The length calculations require scaling laws.
The length of a main burner is primarily based on the distance required for combustion to come to near completion.
Residence time tres in main burner is given by

19. Design Steps: Combustor
Residence time
The reaction time is inversely proportional to the reaction rate and the reaction rate is directly proportional to initial total pressure.

20. Industrial experience shows that

21. Total Pressure Loss in Turbo Combustor
The loss of pressure in combustor (p0,ex <p0,in) is a major problem.
The total pressure loss is usually in the range of 2 – 8% of p0,in.
The pressure loss leads to decrease in efficiency and power output.
This in turn affects the size and weight of the engine.
There are several methods of quantifying the total pressure loss in a combustor,
Relative to the total inlet pressure:
Relative to the inlet Dynamic pressure:
Relative to a reference dynamic pressure:

22. Development of differential Equation for Axial Evolution of Mach Number
Divide throughout by dx
Multiply throughout by M2

23. Differential Equation for Axial Evolution of Mach Number
For a uniform wall heat flux q’’

24. Choking Length with friction
Min

25. Effect of Heat generation on Inlet Conditions
Min

26. Selection of Combustor Inlet Mach Number
Air from the engine compressor enters the combustor at a velocity of about 150 m/s, which is far too high for sustained combustion to take place.
The air is first decelerated to a velocity of about 25 m/s in a pre-diffuser.
The speed of burning kerosene at normal fuel-air ratios is only about 5-10 meters per second; hence any fuel lit even in the prediffused air stream also would be blown away.
Therefore, a region of low axial velocity is created in the combustor, through swirlers so that the flame will remain alight throughout the range of engine operating conditions.

27. Contemporary Main Burners
Engine Type
TF39
Annular
TF41
Cannular
J79
JT9D
F100
T63
Can
Air Flow
(kg/sec)
80.7
61.2
73.5
110
1.5
Fuel Flow
(K/hr)
5830
4520
3790
7300
4800
107
Length (m)
0.53
0.42
0.48
0.45
0.47
0.24
Diameter
(m)
0.85
0.61
0.81
0.965
0.635
0.14
P (kPa)
2630
2160
1370
2180
2520
634
Tcomb oC
1346
1182
927
1319
1407
749